"So whoever of you sights (the crescent on the first night of) the month (of Ramadan, i.e., is present at his home), he must observe Saum (fast) that month... (V.2:185) Sahih al-Bukhari 4506
Arabic hadith text
حَدَّثَنَا عَيَّاشُ بْنُ الْوَلِيدِ، حَدَّثَنَا عَبْدُ الأَعْلَى، حَدَّثَنَا عُبَيْدُ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ ـ رضى الله عنهما ـأَنَّهُ قَرَأَ {فِدْيَةٌ طَعَامُ مَسَاكِينَ} قَالَ هِيَ مَنْسُوخَةٌ.
Translation
Narrated Nafi`
Ibn `Umar recited: "They had a choice, either fast or feed a poor for every day.." and added, "This Verse is abrogated."